Waveguide flanges are used for coupling waveguide sections and waveguide components. When designing waveguide flanges for waveguide joints, consideration is given to the fact that characteristics of waveguide joints affect the mechanical strength and electrical performance of waveguides. For this reason waveguide joints are designed to provide strength and minimize energy reflections and minimal power leakage throughout the frequency range.
Ideally, flat flanges butted together with perfect ohmic contact would produce negligible reflections and negligible power leakage that are frequency insensitive. With a perfect contact-type coupling of flat flanges the waveguide is essentially continuous through the joint. However, a perfect ohmic contact to prevent leakage and reflection requires precise alignment, clean and perfectly flat surfaces and a tight face-to-face surface abutment.
With careful design and assembly, the combined waveguide sections or components are more likely to exhibit desired SWR (standing wave ratio), return loss, reflection and leakage properties over the frequency range. However, flat contact-type flanges cannot tolerate gaps between them and, being susceptible to mechanical vibrations or surface degradation, at higher levels of energy they can produce arcing at the joints. For the same reason, flat contact-type flanges are not suitable for coaxial and rotary joints.
As an alternative, waveguide joints use choke flanges. In a typical configuration, The connection between the waveguide sections is accomplished with a cover flange 14 abutting a choke flange 16 as shown in FIGS. 1a-1c. In the choke flange 16, a circular groove 12 forming a half-wave low-impedance line is inserted, at the joint, in series with the waveguide. The depth of the groove and its radius are each a quarter wavelength (i.e., λ/4) as shown in FIG. 1a. With the quarter wave dimension of the groove the current at the contact points 22 (see FIG. 1a) is substantially zero because any finite resistance at the contact points is in series with infinite impedance. With the dimension of the groove radius being also quarter wave, the impedance at the contact points is substantially zero and provides continuity of the longitudinal current flow between the waveguides sections 18,20 (along the side walls). In other words, because the series line is short-circuited at the far end its input impedance is negligible and the two waveguide sections are essentially continuous through the joint. The actual ohmic contact between the flanges is made at the half-wavelength line where there is a current node and, thus, leakage and energy reflections can be minimized. Additionally, the low characteristic impedance of the half wavelength line over the frequency range reduces frequency sensitivity, but in designing such choke, care must be given to the appropriate wavelength.
FIG. 2 illustrates a coaxial rotary waveguide joint. In its conventional form, a rotary joint is made with a pair of axially aligned flanges and the electrical connection is made with low-resistance contacts. In the illustrated coaxial rotary joint, a DC-blocking connection joins the inner conductors 106, 108 and the outer conductors 102, 104 are joined together by choke-configured connections 112.
However, conventional choke-coupled joints such as those illustrated above require precise alignment and high precision parts. This requirement is particularly important at high frequencies, for example at 38 GHz. For rotary joints the precise alignment prevents return loss and SWR variations and minimizes friction during rotation. To illustrate this point, FIGS. 3a-3b show the cover flange 202 of a choke-coupled joint with spring contacts 222 for mating the waveguides sections 218, 220. These additional components (spring contacts) are necessary to secure ohmic contact between the waveguide sections.